For many decades, acidic clays and amorphous silica-alumina compositions have been used to catalyze the cracking of carbon-carbon bonds, for alkene isomerization and polymerization, aromatic alkylation with alkenes or alcohols, transalkylation, and other acid-catalyzed reactions. The cracking of alkanic bonds requires the highest activity; alkene transformations require lower catalytic strength and/or lower temperature. In the presence of those amorphous porous materials, the catalysis was not shape selective.
ZSM-5 is a porotectosilicate, a microporous crystalline silicate. It is identified by its X-ray diffraction pattern which was described in U.S. Pat. No. 3,702,886, which is relied upon and incorporated by reference herein. It is referred to as a shape selective zeolite. Its shape selective function can be quantified by a test known as the Constraint Index, described below. The shape selectivity is attributed to the pores and channels within the crystal. On the basis of pore size and pore window dimensions, organic compounds or hydrocarbon feeds are provided with constrained access to, and egress from, the internal portion of the crystal. Accordingly, the nature, particularly the size, of molecules which can be subjected to shape selective conversion and of the product(s) which can be produced thereby are controlled or limited by the pore size and pore window dimension of the specific zeolite used in the shape selective catalysis.
Reactions which occur at the surface, rather than within the pores and channels, of the ZSM-5 crystal are not bound by size constraints provided by the pore size and pore window dimensions of the zeolite and, accordingly, tend to be comparable in results to those results produced by amorphous materials used in the past.
The activity of a catalyst comprising ZSM-5 is based inter alia on its acidity. Acidity of a zeolite is a function of the aluminum content of the zeolite. Often the acidity of the zeolite can be gleaned from the determined framework silica:alumina mole ratio.
Various acid catalyzed shape selective reactions can be effected in the presence of the zeolite. These include hydrocarbon cracking, toluene disproportionation, xylene isomerization, alkene conversion, alkene oligomerization, alkene isomerization and methanol conversion (e.g. to gasoline) Cf. W. O. Haag et al, "The active site of acidic aluminosilicate catalysts," pages 589-591, NATURE, Vol. 309 (June 1984).
External acid activity, at the surface of the zeolite crystal, is detrimental to overall selectivity of the shape selective catalytic reactions undertaken in the presence of the zeolite. Accordingly, and for example, in propylene oligomerization to lube range olefins, external surface acid activity is poisoned using bulky organic amines in order to obtain the desired selectivity to near linear lube range olefins.
Zeolite acid activity, sometimes referred as the alpha value, can be increased by various means such as mild steaming, hydrothermal treatment in the presence of alumina, and vapor phase treatment with aluminum chloride. However, such procedures increase acid activity non-selectively and result in materials with increased internal and external acid activity. That increased external acid activity is detrimental to the extent it augments non-shape selective catalyzed product distribution.